A method of producing metal filaments directly from molten metal is an inexpensive method of producing metal filaments. If such metal filaments have an amorphous structure, there would be a great possibility that they could be put into practical use in many applications such as electric and electronic parts, composite materials, and fibrous materials since they have excellent chemical, electrical and physical characteristics. Particularly, in the case of amorphous alloys, the foregoing characteristics can be further improved in comparison with crystal metals and crystal alloys which have heretofore been put into practical use by appropriately choosing the alloy composition. In particular, they have great advantages in corrosion resistance, toughness, and high electromagnetic properties. Thus, there is a great possibility that they are novel materials.
These amorphous metals are already known as described in, for example, Nippon Kinzoku Gakkai Po (Journal of Japanese Metal Association), No. 3, Vol. 15 (1976), Science, No. 8 (1978), and N. J. Grant and B. C. Giessen, Ed., Proceedings of the 2nd International Conference, Elsenier Sequoia S. A., Lausanne (1976).
It has thus been highly desired to produce high quality filaments having a circular cross-section from amorphous metals having such excellent characteristics by a convenient melt spinning method.
Alloys which can be used at present to produce amorphous metal filaments having a circular cross-section by spinning a molten alloy directly into a cooling liquid and solidifying the alloy therein are limited to those having a critical cooling temperature of about 10.sup.3 .degree. C./sec., such as a Pd.sub.77.5 -Cu.sub.6 -Si.sub.16.5 based alloy (atomic %), as described in Scripta Metallurgica, Vol. 13, pp. 463-467 (1979).
The difficulty encountered in making alloys amorphous varies greatly depending on the type of metal and the composition. In particular, an Fe, Ni and Co based alloy which is important as a practical material has a critical cooling rate ranging between about 10.sup.5 .degree. C./sec. and 10.sup.6 .degree. C./sec. and, thereofre, the cooling rate thereof in a cooling liquid is low. It has thus been believed that it is difficult to produce amorphous metal filaments having a circular cross-section from an Fe, Ni and Co based alloy.
At present, for the production of amorphous Fe, Ni and Co based alloy, those methods having a high cooling rate, such as a gun method, a piston-anvil method, a roll chilling method, a centrifugal chilling method, and a plasma jet method, are employed. In accordance with the foregoing methods except for the roll chilling method and the centrifugal chilling method, only amorphous plate-like materials can be obtained. Even using the roll chilling method and centrifugal chilling method, only definite ribbon-like filaments can be obtained, and these filaments have the disadvantage that they cannot be used in other than special applications because of the flat cross-section thereof.
Methods of producing such ribbon-like amorphous metal filaments are known as described in the foregoing literatures concerning amorphous alloys, Japanese patent application (OPI) No. 91014/74 (corresponding to U.S. Pat. No. 3,856,513) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), Japanese patent application (OPI) Nos. 125228/78, 125229/78 and 88219/77, Japanese patent publication No. 50727/77, Japanese patent application (OPI) Nos. 101203/75 and 4017/76, Japanese patent application (OPI) No. 109221/76 (corresponding to German patent application (OLS) No. 2,606,581 and French Pat. No. 2,301,605), Japanese patent application (OPI) Nos. 12719/78 and 12720/78, Japanese patent application (OPI) No. 133826/77 (corresponding to German patent application (OLS) No. 2,719,710 and French Pat. No. 2,350,159), and Japanese patent application (OPI) No. 88220/77.
The conventional method of producing amorphous metal filaments is based on the principle of injecting a molten metal onto the surface of a chilling member, and therefore, the metal filament is inevitably flat at the areas which come into contact with the chilling member, and it has been not possible at all to produce filaments having a circular cross-section. Although an attempt to produce filaments having a circular cross-section by providing the roll surface with round cavities (a continuous narrow cavity having a depth of a several ten .mu.m to a several hundred .mu.m on the roll surface) and injecting a molten metal thereonto was made, there is only a very limited possibility of success since many technical problems arise, for example, it is not possible to inject the molten metal accurately into the very narrow cavity.
A number of methods of producing metal filaments having a circular cross-section directly from a molten metal have been developed.
In accordance with one of the methods, a very instable low viscosity metal stream is cooled and solidified while continuity is retained. That is, this method is based on the same system as melt-spinning which is employed at present for mass-production of synthetic fibers. For example, Japanese patent publication No. 24013/70 discloses, as a stabilization technique for such cooling-solidification, a method in which a molten metal is spun into an atmosphere of a gas reactive with the metal to thereby form an oxidized or nitrided coating film on the molten filament surface. It has been discovered, however, that it is quite difficult to stabilize the molten metal to the same level as in the case of the solidified state only by the formation of such coating films. In addition, this method can be applied only to those specific metals capable of forming oxidized or nitrided coating films.
Japanese patent publication No. 25374/69 discloses a very useful technique for cooling a molten metal. That is, it discloses an important method in which fusing agent particles are sprayed into an ionization region produced by corona discharge such that they float in an inert gas, and the molten metal is cooled and solidified utilizing the latent heat of the fusing agent.
Cooling methods similar to the method disclosed in Japanese patent publication No. 25374/69 are described in, for example, Japanese patent application (OPI) Nos. 56560/73 and 71359/73. In accordance with these methods, a molten metal is spun into bubbles or air bubbles, and cooled and solidified therein. In all of these methods, however, the cooling-solidification rate is very low, and chemical or electrostatic stabilization of a spun stream is still insufficient.
Another cooling method is described in Kasen Geppo (Monthly Reports of Chemical Fibers), No. 7, p. 61 (1974). This cooling method is a composite metal-spinning method utilizing the stringiness of glass, in which a metal such as copper and silver in the form of a chip is placed in a glass tube, and the glass tube and metal are heated and melted with a dielectric heating coil, and withdrawn from a lower portion with a glass rod which has been previously heated and then wound. This composite metal-spinning method, however, is effective only in a specific combination of the melt viscosity of glass and the melting temperature of metal, and is not applicable to all metals. The structure of each of the melting zone and spinning nozzle zone is complicated because of composite spinning and at the same time, high precision is required. Furthermore, when such spun products are used as metal filaments, it is necessary to remove the glass coating film remaining on the periphery thereof. This leads to an increase in production cost, and many problems still remain to be solved before the industrialization thereof.
In addition, a method of producing metal filaments by injecting a spun molten metal into a cooling liquid running in parallel therewith has been proposed as described in Japanese patent application (OPI) No. 135820/74. In this method, however, the cooling ability is, as described hereinafter in detail, insufficient since the spun molten metal and the cooling liquid run in parallel with each other at the same rate and at the same time, at a low rate of 200 m/minute or less. Furthermore, since the cooling liquid is a stream spontaneously falling due to gravity, the impact of the spun molten metal on the cooling liquid, and the boiling and convection of the cooling liquid resulting from the impact make it very difficult to maintain the cooling liquid and the surface thereof in a stabilized state. It is thus not possible to produce high quality amorphous filaments having a circular cross-section. Furthermore, it is technically very difficult to wind up the solidified filament continuously and directly.
Furthermore, a method of producing fine continuous lead wires having a circular cross-section by placing a cooling liquid in a rotary drum, forming a liquid film on the inner walls of the rotary drum by centrifugal force, and jetting molten lead into the liquid film is described in Preliminary Report Title No. 331 at '78 Autumn Conference (No. 83, Toyama) of Japanese Metal Association, and Japanese patent application (OPI) No. 64948/80. This method, however, can be applied only to low melting point metals having good stringiness, such as lead. In particular, under conditions where the jetting rate of the molten metal stream is higher than the rotation rate of the drum, which are described in the literature to be essential in the practice of the method, it is not possible at all to produce high quality fine continuous wires of amorphous alloys. Furthermore, the continuous lead wire produced by the method is not amorphous, has a low cross-sectional roundness (no accurate circular cross-section), is bent, and has high size irregularity in the longitudinal direction. Thus, it is not suitable for practical use.
Using alloys prepared by adding various metalloids or semimetals to Fe, Ni and Co metal elements which are important as practical materials, investigations have now been made to find which metal element exhibits an excellent fine wire-forming ability when it is melted and then solidified by chilling through the introduction thereof into a rotating cooling liquid in a molten form. As a result, it has been found that almost all Ni-based alloys are formed into spherical shots when they are introduced into the rotating cooling liquid, and the fine wire-forming ability thereof is inferior. Furthermore, it has been found that Fe-based alloys which are most inexpensive from the standpoint of starting material costs have an excellent fine wire-forming ability, and that Co-based alloys have a fine wire-forming ability which is slightly inferior to those of the Fe-based alloys.
The term "fine wire-forming ability" as used herein indicates the property of a metal to form uniform continuous filaments having a circular cross-section and without size irregularity in the longitudinal direction when it is spun into a rotating cooling liquid in the form of a molten metal stream and cool-solidified therein.
Hereinafter, the fine wire-forming ability will be described in detail with reference to representative alloys.
It is known that an Ni-Si-B alloy, which is a typical example of Ni-based alloys, very easily provides uniform amorphous continuous flat filaments using a centrifugal chilling method. However, even if the molten metal stream of the Ni-Si-B alloy is spun into a rotating cooling liquid and cool-solidified therein, almost no uniform filament-like product is obtained, and almost all of the molten metal stream is formed into spherical shots.
Also, a Pd.sub.82 -Si.sub.18 alloy (atomic %) having a low critical cooling rate of 1.8.times.10.sup.3 .degree. C./sec. has a poor fine wire-forming ability and when solidified by chilling in a rotating cooling liquid, almost all of the alloy is formed into spherical shots. A Pd-Cu-Si alloy prepared by adding Cu to the above Pd-Si alloy has an excellent fine wire-forming ability, and it is possible to produce therefrom amorphous continuous filaments having a very high uniformity and a circular cross-section. This alloy, however, is very expensive.
Hereinafter, the relation between the fine wire-forming ability and the semimetal contributing to the formation of an amorphous alloy will be explained.
The fine wire-forming ability in a rotating cooling liquid varies markedly depending on the type and combination of semimetal elements. For example, the order of the fine wire-forming ability in a rotating cooling liquid of alloys prepared by adding semimetals to Fe and Co metal elements having an excellent fine wire-forming ability is as follows: EQU Fe-Si-B.gtoreq.Fe-P-Si.gtoreq.Co-Si-B.gtoreq.Fe-P-C
On the other hand, Fe-P-B and Fe-C-B alloys have almost no fine wire-forming ability.
As described above, it is apparent that the fine wire-forming ability in a rotating cooling liquid varies markedly depending on the type of the metal element and semimetal element. Although the reason for this is not at present completely understood, it is believed that the viscosity, surface tension, and cooling rate of the molten metal stream and the physical and chemical action thereof with the rotating cooling liquid are factors.
Furthermore, as in the case of the fine wire-forming ability, the amorphous metal-forming ability varies markedly depending on the type of the semimetal added. In general, the amorphous metal-forming ability increases in the following order: EQU Fe-Si-B.gtoreq.Fe-P-C&gt;Co-Si-B&gt;&gt;Fe-P-Si
On the other hand, with the Fe-P-Si alloy, uniform continuous fine wires can be obtained, but because of the low amorphous metal-forming ability thereof, it is difficult to obtain continuous fine wires which are amorphous.
A method of producing amorphous metal filaments having a circular cross-section using those alloys composed mainly of Fe which is an important material for practical use by jetting an alloy having an amorphous metal-forming ability through a spinning nozzle into a rotating member containing therein a cooling liquid to thereby cool-solidify the spun filament and by winding up the filament onto the inner walls of the rotating member by the rotary centrifugal force of the rotating member wherein the circumferential speed of the rotating member is maintained at the same level as that at which the molten metal is jetted, or alternatively maintained at a higher level than that has been proposed and filed as U.S. Ser. No. 254,714, filed Apr. 16, 1981, now abandoned.
These alloys composed mainly of Fe, however, have disadvantages in that in producing continuous filaments therefrom, problems arise such as plugging of the nozzle and a reduction in the service life of the nozzle during the spinning. In particular, an alloy composed mainly of Fe-P-C tends to be easily oxidized during the spinning and cool-solidification steps. Also, an alloy composed mainly of Fe-Si-B tends to have inferior corrosion resistance. On the other hand, those alloys composed mainly of Co are almost free from the above-described disadvantages, although the fine wire-forming ability and amorphous metal-forming ability thereof are slightly inferior. In particular, they have excellent electromagnetic performance and, therefore, they are useful alloys for the production of electric and electronic parts. Using such useful alloys, however, high quality amorphous metal filaments having a circular cross-section have not yet been produced.